1. Field of the Invention
The present invention relates to a thermal head having a heating element array with a line of a plurality of heating elements, and a thermal printer having the thermal head.
2. Background Arts
A thermal printer is known to record images on a recording paper by using a thermal head having a line of a plurality of heating elements (resistors). As the thermal printers, there are a color direct thermal printer that develops colors by directly heating a thermosensitive recording paper by means of the heating elements, and a thermal transfer printer that transfers the ink of the ink ribbon to the recording paper by putting the ink ribbon on the recording paper and heating the rear face of the ink ribbon by means of the heating elements.
Japanese Patent Laid-Open Publication Number H10-138543 discloses a thermal head that has the heating elements comprising first heating resistors disposed at the upstream side of a transportation path of the recording paper and second heating resistors disposed at the downstream side of the first heating resistors. The first heating resistors supply the recording paper with a minimum energy necessary to develop color, while the second heating resistors supply the recording paper with the energy to develop color at predetermined gradations. This thermal head is capable of reducing the driving electrical current to be supplied to the second heating resistors.
Another thermal head is capable of extending the lives of the heating elements by having the first heating resistors and the second heating resistors, which are disposed at a predetermined distance from the first heating resistors in the transporting direction of the recording paper, and using them alternately (for example, disclosed in Japanese Patent Laid-Open Publication Number H10-278329). One of the methods for recording fine lines and minute dots with the above thermal heads is to partially heat the recording paper by using the heating elements shortened in the transporting direction of the recording paper. However, the thermal head, disclosed in Japanese Patent Laid-Open Publication Number H10-138543, is not capable of recording fine lines and minute dots, regardless of that the first and the second heating resistors have different lengths. This is because this thermal head performs color developing twice, first with the first heating resistors then with the second heating resistors. The thermal head disclosed in Japanese Patent Laid-Open Publication Number H10-278329 may extend the lives of the heating elements, but it is not capable of recording fine lines and minute dots either, for having the heating elements of the same length.
Although the thermal head disclosed in Japanese Patent Laid-Open Publication Number H10-138543 is possibly capable of recording fine lines and minute dots by selectively heating the first heating resistors that is shorter than the second heating resistors, it needs to have electrode layers threading between the first and the second heating resistors. This results not only in complicating the constitution of the thermal head that leads to increase the manufacturing cost, but also in inhibiting the downsizing of the thermal head because the adjacent heating elements need to be spaced to each other.